Most brushware, especially toothbrushes, are produced via a two-part technique. The handles are produced by injection molding, following which the bristles are inserted into the handle. The most common technique for inserting the bristles into the brushes is stapling. The bristles are folded around a metal staple which is pushed into a pre-molded hole in the brush. The staple cuts into the plastic at the periphery of the hole, and the plastic retains both it and the bristles. However, it does not require great force to remove the bristles from the handle. Vigorous brushing can easily cause the bristles to be removed from the handle, leading to shedding or even release of the metal staple inside of the mouth.
Techniques wherein the bristles are fused with the handle can be used to produce brushes from which the bristles are not so readily removed. Either the bristles or the brush head, or both, are heated, and the bristles are inserted into holes in the handle where they are retained by the cooling plastic. Exemplary techniques for brush production by fusion include those described in U.S. Pat. No. 4,988,146, which describes a fusion process wherein the ends of bristle bundles are thermally fused, shortening and locally thickening the bundles to form a fuse-ball, or fuse. The fused bristles are inserted into holes in a brush handle which have a smaller cross-section than the fuse-ball. Either the fuse-ball or the wall of the hole may be heated to allow the fuse-ball to conform to the inside of the hole, or the bristles may be inserted into the hole immediately following fusion, before the fuse-ball is allowed to cool.
U.S. Pat. No. 5,224,763 discloses a fusion process in which holes are formed in the handles during the injection molding process. A collar of excess plastic disposed about the hole is swaged around the fused end of the bristles when it is inserted into the hole.
U.S. Pat. No. 5,622,411 discloses a fusion process wherein it is assumed that the fused bristles will displace a finite amount of material when they are inserted into holes in the handle head. The displaced material is compressed to form a planar surface in the head of the brush.
In each of these techniques, the inventor has sought a method of inserting bristles into pre-cored holes in the handle. However, it is not necessary to use a handle with prefabricated holes; the holes can be formed immediately prior to the insertion of the bristles. In a process called "hedgehogging," the handle is heated, and a set of short spikes mounted on a heated plate is pushed into the head of the handle to form holes to receive the bristles. U.S. Pat. No. 4,637,660 describes an exemplary hedgehogging process wherein, as the holes are formed in the handle, the displaced material is organized into a small bead surrounding the newly-formed hole. As in the '146 patent, the bristle ends are fused before they are inserted into the hedgehogged holes. Material from the small bead flows around the fuse after it is fitted into the hole, enclosing the bristles in the brush head.
In most of these methods, a significant portion of the brush must be heated before the bristles are inserted into the hole whether it is pre-cored or hedgehogged. The only exception is the '146 patent, where the heated fuses may be inserted into the hole without heating the handle. In this case, the fuse expands against the walls of the hole, thermoforming the fuse and exerting hydrostatic pressure on the walls of the hole. In either case, excess post-molding processing of the handle will weaken the head and may cause it to warp. In addition, the fracture toughness of the head may be reduced. Many users bang their toothbrushes against the side of the sink to remove excess water after they are done brushing their teeth. This action is more likely to break a warped or pre-stressed brush head.